nixos/dbus: explicitly set homeMode for dbus

Otherwise /run/dbus is created with the wrong permissions.

For some reason our recent changes made /run/dbus be 0700 instead of
0755. This is actually the default of homeMode. So something worked by
accident before?

nixos/modules/services/system/dbus.nix

@@ -95,6 +95,7 @@ in
         uid = config.ids.uids.messagebus;
         description = "D-Bus system message bus daemon user";
         home = homeDir;
+        homeMode = "0755";
         group = "messagebus";
       };